Rack bars, e.g., rack bars for a steering apparatus of a vehicle, are formed from a hollow metal pipe. For example, a portion of a steel pipe where a rack toothed portion is to be formed is flattened to have a flat outer surface. Then, a female die having a profile corresponding to rack teeth to be formed is pressed against the flattened portion, and a mandrel is inserted into the steel pipe.
By inserting the mandrel into the steel pipe, the flattened portion is pushed outward from the inside and is plastically deformed. With the female die being pressed against the flattened portion, the teeth are formed in accordance with the profile of the female die.
To prevent a burr from being generated when forming the rack teeth by plastic working using the mandrel, the portion of the steel pipe where the rack toothed portion is to be formed may be formed to have a flat portion having the flat outer surface and slanted portions on respective sides of the flat portion (see, e.g., JP 2002-178095 A).
When forming the rack teeth by plastic working from the inside of the pipe member while pressing the female die against the pipe member, the rack teeth are formed to protrude up from their central portions in a widthwise direction of the rack teeth. The widthwise direction of the rack teeth is a direction intersecting the direction in which the rack teeth are arranged side by side.
The width of the rack teeth is increased in proportion to the number of times the mandrel is inserted into the pipe member. Therefore, the number of times the mandrel is inserted into the pipe member is adjusted so that the rack teeth have a required width.
As described above, the rack teeth are formed to protrude up from their central portions in the widthwise direction, and the mandrel is inserted into the pipe member multiple times until the rack teeth have the required width, which requires a certain amount of time.
Further, because the rack teeth are formed to protrude up from their central portions in the widthwise direction, a load is repeatedly applied to a portion of the die that is brought into contact with the central portions of the rack teeth, which may shorten the life of the die.